NOTES
2678
Results and Discussion Gas-Phase Decomposition. A series of runs were carried out a t 162". The noncondensable fraction contained no methane and consisted entirely of nitrogen. Analysis of the fraction volatile a t -121" showed it to contain only about 3% ethane, the remainder being carbon dioxide with traces of the azodiformate ester. A typical product analysis yielded the following values (in micromoles): N2, 205; COz, 1.94; and C2H6, 0.06; i.e., N2/COz = 1.06 and 2C2Hs/ COZ = 0.06. CH300CN=NCOOCH3
Nz
CH300c +CHa
+ 2CH300e
+ CO:!
(5) (6)
Decomposition of the ester according to reactions 5 and 6 would require that the ratios N2/C02 and ZCH,. / COz would have the values 0.5 and 1, respectively. We interpret the divergence of our experimental values from these predicted values to indicate that only about half of the methoxycarbonyl radicals generated in reaction 5 decompose. It appears that approximately 6% of the methyl radicals produced in the decarboxylation reaction are accounted for by the reaction 2CH3 '
4CzH6
(7)
Methyl radicals have been shown to react very readily with azomethanes by addition to form trimethylhydrazine and tetramethylhydrazine. Our experimental results suggest that they may also react very readily with the azodiformate. It is likely also that some of the unaccounted for methoxycarbonyl radicals have reacted by addition to the K=N bond. Conjugation with the two carbonyl groups appears to have the effect of enhancing the rate of radical addition to the double bond. This reaction must be fast, since it is clearly competitive with reaction 6 and very much faster than the hydrogen atom abstraction reaction
CH3.
+ CHaOOCN=NCOOCH3 CH,
CH300CfiTN(R)COOCH3 --3 CH,OC(O)=NN(R)COOCH3 This radical in turn reacts by addition to the azodiformate and in this manner a repeating unit (OX(COOCH3)N=C(QCH3)-), is built up. Our molecular weight determination suggests a value of about 3 or 4 for n. This ready addition reaction rapidly consumes all of the azodiformate and this explains why the solution is bleached, although actual thermal decomposition of dimethyl azodiformate based upon nitrogen evolution only accounts for a relatively small consumption of the ester. It is clear that because of this complication the decomposition of azodiformate esters is not likely to be of use as a free-radical source.
Acknowledgment. We thank Professor Peter Gray for several helpful discussions. (9) M. H. Jones and E. W. R. Steacie, J . Chem. Phys., 21, 1018 (1953). (10) A. Good and J. C. J. Thynne, J . Chem. Sac., B , 684 (1967).
+
+ *CHzOOCnT=IYCOOCHs
(8)
because no methane is detected. Rate constants (based on nitrogen evolution) were measured for reaction 5 at 162". The results of five repeat determinations yielded a value k; = (6.4 It 0.9) X sec-l, the error limit representing the average deviation of these runs. If a preexponential factor of 1014 sec-l is assumed, this rate corresponds to an activation energy for reaction 5 of about 38 kcal mol-'. This may be compared with a value of 34.6 kcal mol-' for the activation energy reportedlo for the decomposition of tetramethyltetrazene, Le., where R is (CH3),N. in reaction 1. Decomposition in Dodecane Solution. When M solutions in dodecane were decomposed thermally a t temperatures in the range 120-170" or photochemically T h e Journal of Physical Chemistry
a t a lower temperature, a colorless sticky polymerlike solid was produced which went brown on standing. The bright red solution was rapidly bleached, suggesting complete consumption of the azodiformate, although the maximum yield of nitrogen indicated that only about 7% of the azodiformate had decomposed, Analysis of the polymerlike solid showed it to contain %, 44.1%; N, 12.6%; H, 6.4%; and 0, 36.8% (by difference). This corresponds to a formula of C17H30OllN4, the molecular weight of which is 466, in reasonable agreement with the molecular weight of 500 i 20 determined directly using a vapor pressure osmometer. We consider that these results may be interpreted in terms of a ready addition of the radicals produced in reactions 5 and 6 to the azo linkage. The resulting radical then undergoes an isomerization reaction
Some Observations on the Proton Magnetic Resonance Spectrum of Tetraethyl Ethylenebisphosphonate
by M. P. Williamson and C. E. Griffin Department of Chemistry, Unizersity of Pittsburgh, Pittsburgh, Pennsyluania, and Mellon Institute, Carnegie Mellon University, Pittsburgh, Pennsylzania (Receiued February 23, 1968)
The salient features of the proton magnetic resonance spectra of a number of compounds containing the PCHzCH2Psystem (1) have been reported recently.lt2 (1) A. J. Carty and R. K. Harris, Chem. Commun., 234 (1967). (2) J. J. Brophy and M. J. Gallagher, Austral. J . Chem., 20, 503 (1967).
NOTES
2679
For compounds in which the phosphorus atoms are symmetrically substituted, the methylene protons yield a deceptively simple spectrum which has been treated as being due to the X part of an AXX'X''X'''A' spin system.l The spectrum normally observed is a triplet with the separation of the relatively sharp and intense outer lines equal to I 2 J p ~4- 3 J P H ' 1 although the appearance of the spectrum is markedly dependent upon the nature of the substitution at phosphorus. The structure of the inner peak varies erratically from a sharp singlet to a broad singlet and even to a partially resolved multiplet.1~2 Partial analyses of the spectra of a number of compounds of type 1 have been reported by Carty and Harris.l In an effort to obtain a complete analysis of one representative of this class, we have examined the pmr spectrum of tetraethyl ethylenebisphosphonate [ (CH3CHzO)zP(O)CHzCHzP(O)(OCHZCH3)2 (2)] in some detail.
Experimental Section Samples of 2 were kindly provided by Drs. D. J. Martin and R. L. K. Carr. The samples were degassed on a vacuum line and sealed in 5-mm nmr tubes after the addition of 3% tetramethylsilane (TMS). The decoupling experiment was performed on a Varian HA-100 spectrometer which was frequency swept in the HA mode, the field frequency being locked to the internal TRW. The irradiating frequency was provided by a Hewlett-Packard 201CR audiooscillator. All the other spectra were recorded on a Varian A-60 spectrometer. The high- and low-temperature measurements were made using a Varian V-4060 variabletemperature controller and a V-6031B probe.
I
ik
Discussion The -CH2CH2- portion of the pmr spectrum of 2 in carbon telrachloride [25% (v/v) solution] at 37" is^ shown in Figure lb. A spectrum of similar appearance has been reported by Brophy and GallagheraZ The observed separation (9.0 Hz, lit.2 9 He) of the two intense lines is of the expected magnitude for I 2 J p ~ Several trial calculations of the spectrum were performed using a program written specifically for the AXX'XI'X' ''A' ~ y s t e m . ~It became apparent that the observed broadness of the lines was not being matched. This broadness was neither dependent on the source of the sample used nor could it be attributed to the viscosity of the sample; the broadness was found t o persist even in dilute solutions. The resonances of the ester ethyl groups were also found to be unusually broad, particularly in the methylene region. It was thought that the broadness of the -CHzCHz- region could have been due to either a mixing of the spin states of the phosphorus nuclei by coupling with the -OCH,protons of the ethyl group, or possibly, a proton-proton coupling ( 5 J H H ) between -CHzCH2- and -OCHz- protons. Brophy and Gallagher2 have indicated longer
+
Figure 1. T h e PCHnCH2P region of t h e pmr spectrum of tetraethyl ethylenebisphosphonate as a 257, (v/v) solution: (a) i n nitrobenzene at 180'; ( b ) in carbon tetrachloride at 37'; (0) in carbon disulfide a t -50'.
range couplings t o be a possible cause of line broadening in spectra of compounds of type 1. I n order to test this postulation, a double-resonance experiment was carried out. Irradiation of the -OCHz- region of the spectrum at 100 MHz failed to affect the appearance of the -CHzCHz- resonances; no line narrowing was observable. Under these conditions, the ester methyl resonance collapsed to a singlet, indicating that the ester methylene protons had, in fact, been decoupled. A second possible explanation of the broadness of the lines in the spectrum of 2 was a lack of rotational averaging. The bulk of the (CH3CHZO),P(O)-groups might be sufficient t o cause a slow rotation about the (3) A. A. Bothner-By, personal communication. The input data for this program accommodate different values for JHCCH gauche and
trans.
Volume 72. Number 7 Julu 1968
NOTES
2680 methylene carbon-carbon bond. In this event, it would be expected that the lines would narrow with an increase in temperature. However, even at 180" the spectrum of 2 in nitrobenzene [25% (v/v) solutions] failed to show any detectable line narrowing in either the -CH2CH2- (Figure la) or the ester regions. If the premise of slow rotation were correct, this finding would indicate the barrier to rotation t o be extremely high, In such a case, it should have been possible to freeze out one of the rotamers at low temperature. However, the spectrum of 2 in carbon disulfide [25y0 (v/v) solution] at -50" (Figure IC) showed no line narrowing. The effect of temperature on the spectrum of 2 is summarized in Table I. Further, the appearance of the spectrum changed only slightly on passing from the neat liquid to dilute solutions in carbon tetrachloride, carbon disulfide, nitrobenzene, acetone-&, and trifluoroacetic acid. Previous investigations have shown dialkyl alkylphosphonates to possess a modest solvent dependence, particularly for solutions in trifluoroacetic acids4 These changes may be the result of changes in rotamer populations.
Table I : Nmr Spectra of Tetraethyl Ethylenebisphosphonate
+
IWH Solvent
Concn, % (v/v)
Temp,
SJPHl
"C
Hz
CC14
25
37
9 0
Acetone-ds
12.5
37
8 8
Trifluoroacetic acid
25
37
8.3
PhNOz
25
37 100 150 180
8.4 8.1 7.6 7.3
cs2
10 25
37 37 - 10 - 50
8.7 8.8 9.0 9.3
Although the results of this study are inconclusive, some possible explanations for the appearance of the spectra of symmetrical PCHzCHgP compounds are eliminated. A complete analysis of the spectra of this system must probably await the availability of stereospecifically deuterated compounds.
Acknowledgments. The authors are indebted to Professor A. A. Bothner-By for helpful discussions and to the Hooker Chemical Corp. for a generous supply of one of the samples of tetraethyl ethylenebisphospho(4) M. Gordon, Ph.D. Thesis, University of Pittsburgh, Pittsburgh, Pa., 1965. The Journal of Physical Chemistry
nate. This work was performed using, in part, the nmr facility for biomedical studies supported by a Grant (FR 00292) from the National Institutes of Health.
2-Aminopyridine as a Standard for Low-Wavelength Spectrofluorimetry
by R. Rusakomicz and A. C. Testa Department of Chemistry, St. John's University, Jamaica, New York 11&% (Received March 5 , 1968)
The use of fluorescent compounds to standardize photomultiplier-monochromator assemblies is commonplace in many laboratories today. In general, convenience dictates the use of such compounds. Quinine bisulfate (QBS) and A1-PBBR chelate have been used as reference compounds;'j2 however, these compounds fluoresce above 390 mp. Fluorescence in the lowwavelength region (300-400 mp) has been given very little attention with regard to common acceptance of a reference compound. I n view cf this shortcoming, we report the results of experiments with 2-aminopyridine (AMP) in 0.1 N H&o4 and suggest that it is a convenient fluorescence standard in the wavelength range 315-480 mp. Illustrated in Figure 1 is the single-peak envelope of the fluorescence spectrum of 2 A I " in 0.1 N HzS04, resulting from excitation at 285 mp. The data presented are an average of the emission spectra obtained at concentrations of 1.25 X 10-Eand 4.53 X M and have been normalized to the fluorescence wavelength maximum at 367 mp. The recorded fluorescence spectrum has been corrected for the wavelength sensitivity of our photomultiplier-monochromator system, which has been calibrated with a standard lamp of known irradiance (pLW/cm2 see 10 8). It is evident that there is no significant difference between the corrected and recorded fluorescence s p e ~ t r u m . ~ Since this is obviously an advantage in calibration, it is also convenient for quantum-yield determinations. An estimate of the difference between the area of the recorded and corrected spectra is f 10%. The determination of the fluorescence quantum yield of 2-AMP was made by comparison to QBS in 0.1 N HzS04. In order to justify 285-mp excitation for QBS rather than the generally used 366-mp excitation,' we determined the fluorescence area of QBS at these two excitation wavelengths. Although there is a small (1) W. H. Melhuish, J . Phys. Chem., 64, 762 (1960). (2) R. J. Argauer and C. E. White, Anal. Chem., 36, 368 (1964). (3) Calibration of the 1P21 photomultiplier-monochromator system with a standard lamp in our laboratory indicated that the correction factors for wavelength sensitivity of the tube were unity in the region 316-360 mfi,
